QS-21 is a potent vaccine adjuvant currently sourced by extraction from the Chilean soapbark tree. It is a key component of human vaccines for shingles, malaria, coronavirus disease 2019 and others under development. The structure of QS-21 consists of a glycosylated triterpene scaffold coupled to a complex glycosylated 18-carbon acyl chain that is critical for immunostimulant activity. We previously identified the early pathway steps needed to make the triterpene glycoside scaffold; however, the biosynthetic route to the acyl chain, which is needed for stimulation of T cell proliferation, was unknown. Here, we report the biogenic origin of the acyl chain, characterize the series of enzymes required for its synthesis and addition and reconstitute the entire 20-step pathway in tobacco, thereby demonstrating the production of QS-21 in a heterologous expression system. This advance opens up unprecedented opportunities for bioengineering of vaccine adjuvants, investigating structure–activity relationships and understanding the mechanisms by which these compounds promote the human immune response.

QS-21 是一种强效疫苗佐剂，目前从智利肥皂树中提取。它是带状疱疹、疟疾、2019 年冠状病毒病和其他正在开发的人类疫苗的关键成分。 QS-21 的结构由与复杂的糖基化 18 碳酰基链偶联的糖基化三萜支架组成，该链对于免疫刺激活性至关重要。我们之前确定了制造三萜糖苷支架所需的早期途径步骤；然而，刺激 T 细胞增殖所需的酰基链的生物合成途径尚不清楚。在这里，我们报告了酰基链的生物起源，表征了其合成和添加所需的一系列酶，并重建了烟草中的整个 20 步骤途径，从而证明了 QS-21 在异源表达系统中的生产。这一进展为疫苗佐剂的生物工程、研究结构-活性关系以及了解这些化合物促进人类免疫反应的机制开辟了前所未有的机会。